This invention relates to a process for the conversion of hazardous hexafluoroarsenic acid or any salt thereof contained in an aqueous mixture to a form that can be made nonhazardous. More particularly, this invention provides a process for converting hazardous hexafluoroarsenic acid (HAsF.sub.6) or any salt thereof such as potassium hexafluoroarsenate (KAsF.sub.6), sodium hexafluoroarsenate (NaAsF.sub.6), ammonium hexafluoroarsenate (NH.sub.4 AsF.sub.6), calcium hexafluoroarsenate (Ca(AsF.sub.6).sub.2), and magnesium hexafluoroarsenate (Mg(AsF.sub.6).sub.2) contained in an aqueous mixture to arsenic acid (H.sub.3 AsO.sub.4) or any salt thereof which can then be rendered nonhazardous by the best developed available technology.
A method generally employed in the manufacture of hydrogen fluoride involves heating a mixture of fluorspar and sulfuric acid in a rotating furnace; see for example commonly assigned U.S. Pat. No. 3,718,736. The crude hydrogen fluoride gases leaving the furnace are scrubbed to remove entrained solids, cooled, and condensed to form an initial crude product. The initial crude product formed which comprises at least 95 percent by weight of anhydrous hydrogen fluoride contains various undesirable impurities and these are removed by fractional distillation to give technical or industrial grade anhydrous hydrogen fluoride which has a purity of 99.95% hydrogen fluoride or better. The industrial grade anhydrous hydrogen fluoride thus obtained still contains large quantities of undesirable impurities such as arsenic because the starting material, fluorspar, contains arsenic, and this arsenic cannot be removed in the distillation process. The amount of arsenic impurity which is present in industrial grade anhydrous hydrogen fluoride depends on the arsenic impurity in the fluorspar from which anhydrous hydrogen fluoride is produced. The industrial grade anhydrous hydrogen fluoride generally contains about 50-500 ppm of arsenic impurity.
The presence of arsenic impurity in anhydrous hydrogen fluoride at these levels is highly undesirable for many applications. Anhydrous hydrogen fluoride is used in the refining and chemical manufacturing industries and arsenic impurities in anhydrous hydrogen fluoride can poison the catalyst and contaminate the manufactured product which adversely affects the product quality. In the electronics industry, aqueous solutions of hydrogen fluoride are used as cleaning agents and etchants in the manufacture of semiconductors, diodes, and transistors. A high degree of purity and very low levels of arsenic in anhydrous hydrogen fluoride are required to prevent minute quantities of arsenic impurity from remaining on the surfaces of the electronic industry products after they have been cleaned or etched with hydrogen fluoride. Furthermore, arsenic in anhydrous hydrogen fluoride can ultimately cause an environmental problem for the end user.
Commonly assigned U.S. Pat. No. 4,756,899 provides a process for manufacturing high purity anhydrous hydrogen fluoride having low levels of arsenic impurity by contacting anhydrous hydrogen fluoride product, or an intermediate product obtained during the manufacture of anhydrous hydrogen fluoride with hydrogen peroxide in the presence of a catalyst which comprises effective amounts of molybdenum or an inorganic molybdenum compound and a phosphate compound. The volatile trivalent arsenic impurity in the anhydrous hydrogen fluoride is oxidized to a nonvolatile pentavalent arsenic compound and the resultant mixture is distilled to recover high purity anhydrous hydrogen fluoride with reduced levels of arsenic impurity.
Commonly assigned U.S. Pat. No. 4,929,435 provides a process for manufacturing high purity anhydrous hydrogen fluoride having low levels of arsenic impurity by contacting anhydrous fluoride product, or an intermediate product obtained during the manufacture of hydrogen fluoride, with hydrogen peroxide to oxidize the arsenic impurity in the presence of a catalyst which comprises an effective amount of a component selected from the group consisting of an organic molybdenum compound, vanadium, and a vanadium compound, and a phosphate compound. The volatile trivalent arsenic impurity in the anhydrous hydrogen fluoride is oxidized to a nonvolatile pentavalent arsenic compound and the resultant mixture is distilled to recover high purity anhydrous hydrogen fluoride with reduced levels of arsenic impurity.
U.S. Pat. No. 4,954,330 provides a process for manufacturing purified anhydrous hydrogen fluoride having reduced levels of arsenic impurity by contacting anhydrous hydrogen fluoride with an effective amount of hexavalent chromium oxide and oxygen. The volatile trivalent arsenic impurity in the anhydrous hydrogen fluoride is oxidized to a pentavalent arsenic compound and the resultant mixture is distilled to recover anhydrous hydrogen fluoride with reduced levels of arsenic impurity.
U.S. Pat. No. 4,032,621 provides a process for manufacturing purified anhydrous hydrogen fluoride having reduced levels of arsenic impurity by contacting anhydrous hydrogen fluoride with an oxidizing agent such as potassium permanganate and then with a reducing agent such as hydrogen peroxide. The impurities are converted to residues with low volatility compared to anhydrous hydrogen fluoride and the resultant mixture is distilled to recover anhydrous hydrogen fluoride with reduced levels of arsenic impurity.
Each of the four preceding processes and others involve a distillation step wherein the purified anhydrous hydrogen fluoride is separated from the impurities by distillation. These impurities plus some anhydrous hydrogen fluoride collect in the bottom of the distillation column. A typical make-up of such a mixture which collects in the distillation column bottom is about 75 to about 95 percent by weight hydrogen fluoride, about 2 to about 20 percent by weight water, up to about 5 percent by weight sulfuric acid, and up to about 5 percent by weight hexafluoroarsenic acid or salt thereof.
This typical distillation column bottom has presented the following two problems to the industry. First, the waste contains hexafluoroarsenic acid or salts thereof which cannot be rendered nonhazardous with current stabilization technology. The current practice is to purify a small portion of the total production and either recycle the distillation bottoms to the process or ship the distillation bottoms to a hazardous waste site. The first option reduces the capacity for purified hydrogen fluoride while the second option is expensive and may not be allowed without pretreatment.
Second, hydrogen fluoride will be used in the manufacture of hydrochlorofluorocarbons and hydrofluorocarbons which are considered to be substitutes for the currently used chlorofluorocarbons suspected of detrimentally affecting the ozone layer. As an example of such a manufacturing process, hydrogen fluoride is reacted with vinylidene chloride or 1,1,1-trichloroethane to form 1,1-dichloro-1-fluoroethane (which is known in the art as HCFC-141b). As a result of this increased demand for higher purity hydrogen fluoride, manufacturers seek a means for recovering substantially all of the hydrogen fluoride in the distillation column bottom so as to maximize their yield of purified hydrogen fluoride and minimize production costs.
It has been attempted to convert the hexafluoroarsenic acid or salt thereof to a nonhazardous material but the following problems have been encountered. The hexafluoroarsenate ion is extremely stable and hexafluoroarsenic acid and salts thereof are generally soluble as discussed by Clark et al., "Ligand Substitution Catalysis via Hard Acid-Base Interaction," J. of Am. Chem. Soc. 92(4), 816 (1970) and Christe et al., "Novel Oxonium Salts Preparation and Characterization etc.", Inorg. Chem. 14 (9), 2224 (1975). Attempts have been made to convert the hexafluoroarsenate ion to a nonhazardous material by hydrolysis. Lockhart, Dissertation, Vanderbilt University, 1967, reports on hydrolysis rate studies for dilute hexafluoroarsenate ion in strong sulfuric acid, about 70%. The initial reaction rate is reported to be fast but then it decreases. At high levels of sulfuric acid, the reaction rate decreases indicating an insufficiency of water required for the hydrolysis reaction; at low levels of sulfuric acid, the reaction rate is very slow indicating insufficient sulfuric acid to carry out the required catalysis. This process is commercially unattractive because the reaction rate slows as the process reaches an equilibrium.
Lockhart et al., J. Inorg. Nucl. Chem. 31, 407 (1969) reported on the acid hydrolysis of hexafluoroarsenate ion. The article concluded that as the sulfuric acid concentration increases from 45 to 85%, the hydrolysis rate increases, but since the reaction is reversible, ultimately an equilibrium amount of unhydrolyzed hexafluoroarsenate ion is reached. Regardless of the sulfuric acid concentration, time, and temperature studied, an equilibrium occurs wherein at least 150 parts per million of hexafluoroarsenate ion are present. This level of hexafluoroarsenate ion is in excess of that which would allow the resulting material to be rendered nonhazardous.
As such, the need exists in the art for a process which converts substantially all of the hazardous hexafluoroarsenic acid and salts thereof to a form that can be rendered nonhazardous, proceeds in a reasonable amount of time under reasonable conditions, and provides for the recovery of the hydrogen fluoride if desired.